The present invention relates generally to apparatus for maintaining a load of a closed cycle helium refrigerator substantially constant and, more particularly, to such an apparatus wherein temperature variations of the refrigerator and load are compensated by sensing the temperature variations and supplying compensating, oscillating thermal energy to the refrigerator and the load.
In certain very high resolution infrared spectroscopy applications, it is necessary to provide an infrared source having a very stable output frequency. In one particular application, it is desired to provide a source that generates optical lines over only a 5 mHz band width. The resolution required to achieve such a band width can be appreciated when it is realized that the frequency of a typical IR emitter is between approximately 10.sup.12 and 10.sup.14 Hz.
It has been proposed to employ semiconductor diode lasers as high resolution infrared sources in connection with high resolution infrared spectroscopy devices. However, such sources have an output that is relatively dependent upon the temperature of the semiconductor diode. One particular type of semiconductor diode laser has been found to have a tuning rate of 30 gHz per degree Kelvin, i.e., the output frequency of the laser shifts 30 gHz for each degree Kelvin change in the temperature of the laser. To stabilize the output frequency of such a laser to 5 mHz, it is necessary to stabilize the temperature of the laser to within 1.6 .times. 10.sup.-4 .degree.K.
A device that has been frequently employed in the past to stabilize the temperature of semiconductor diode IR lasers has been a closed-cycle helium refrigerator. By mounting the diode laser on a cold tip of the closed cycle helium refrigerator, the temperature of the diode is maintained to approximately 0.4.degree. K., which results in an output band spread of approximately 12 gHz. One of the reasons why there is a relatively large temperature variation in the diode laser is because the closed cycle helium refrigerator periodically pumps helium at 3 Hz to cause a cyclic rate of heat transfer to the cold tip and the diode.
To reduce the 0.4.degree. K. variation substantially, the sample has been isolated from the cold tip with a temperature attenuator formed of a layer of a material or combination of materials which effectively attenuate the amplitude of the temperature oscillations occurring due to the cyclic nature of the refrigerator. The temperature attenuator has the disadvantage of causing the minimum temperature of the thermally isolated diode to be always higher, by a few degrees Kelvin, than the temperature of the cold tip. If there is not a significant temperature difference between the cold tip and the diode, there is not a significant enough reduction of the amplitude of the temperature oscillations. However, the use of a large quantity of the attenuator materials causes the thickness of the attenuator to be increased until the temperature of the diode may not be low enough to emit wave lengths of interest. Hence, in actuality the thermal attenuator, while it provides the desired compensation, is not wholly satisfactory because the desired temperature range for the laser may not be achieved.